Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual é o usuário registrado S, quando Userpics Free é 6 [grátis] ou 15 [plus]?
 ###Tabelas: CREATE TABLE ptv_stations_nationwide (ptv_stations_nationwide_id AUTO_INCREMENT PRIMARY KEY, Branding VARCHAR, Callsign VARCHAR, Ch. # VARCHAR, Power kW VARCHAR, Station Type VARCHAR, Location (Transmitter site) VARCHAR) CREATE TABLE standard_parameters (standard_parameters_id AUTO_INCREMENT PRIMARY KEY, Region VARCHAR, Start datum VARCHAR, Target datum VARCHAR, c_x ( Metre ) VARCHAR, c_y (Metre) VARCHAR, c_z (Metre) VARCHAR, s ( ppm ) VARCHAR, r x ( Arcsecond ) VARCHAR, r y ( Arcsecond ) VARCHAR, r z ( Arcsecond ) VARCHAR) CREATE TABLE north_group (north_group_id AUTO_INCREMENT PRIMARY KEY, Team VARCHAR, Match VARCHAR, Points VARCHAR, Draw VARCHAR, Lost VARCHAR) CREATE TABLE general_information__&__feature_comparison (general_information__&__feature_comparison_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Year Began VARCHAR, s Registered user VARCHAR, Updated in past 30 days VARCHAR, Registration VARCHAR, Ability to syndicate off-site feeds VARCHAR, Userpics Free VARCHAR, Userpics Paid VARCHAR, Monthly Cost for Paid Account VARCHAR, Yearly Cost for Paid Account VARCHAR, Permanent Account VARCHAR) CREATE TABLE doubles:_6_5_titles_1_runner-up (doubles:_6_5_titles_1_runner-up_id AUTO_INCREMENT PRIMARY KEY, Outcome VARCHAR, Date VARCHAR, Tournament VARCHAR, Surface VARCHAR, Partner VARCHAR, Opponent VARCHAR, Score VARCHAR)  
SELECT s Registered user FROM general_information__&__feature_comparison WHERE Userpics Free = 6 [free] or 15 [plus]